An intake muffler of the above species is known from German patent document 743,418 A. FIG. 1 of this document shows, immediately after the intake-air filter, a muffler system wherein the intake pipe is fitted with wall apertures directly behind the dust-filter cartridge and is enclosed by an insulating material. After this segment, as seen in the direction of flow, the intake pipe comprises an axial sequence of apertures arrayed in an annular manner. Each aperture communicates with a chamber externally enclosing the intake pipe. The intake-air acoustic waves passing through any particular set of apertures are deflected by bell-shaped sleeves so that they move in counterflow to the intake-pipe airflow into the enclosing chamber and onto the radially directed chamber base. The chamber base acts as a reflector. As a result, the back-reflected acoustic waves moving towards the set of intake apertures effectively lower the acoustic admittance of the apertures in the intake pipe such that effectively coupled intake-muffling is achieved even at frequencies in the lower-frequency range.
Accordingly the state of the art disclosed in the '418 German reference discloses intake muffling by resonance-coupling of a reflection damper. Dampers of this construction are suitable only at low frequencies, for instance to dampen 2nd-order engine vibrations, since they comprise only a single resonance frequency per chamber. Moreover, they entail bulk due to the configuration of each reflecting chamber and the number of chambers required to achieve at least a moderately broad band in the intake muffler.
Another muffler for internal-combustion engines is known from German patent document 580,923 A in the form of an intake muffler or backfire damper. Boreholes determining resonance wavelengths in the exhaust pipe are enclosed by a sleeve because of the higher pressure in the exhaust pipe. This sleeve degrades the damping of the exhaust muffler.
Another intake muffler is known from U.S. Pat. 4,350,223. This intake muffler is inserted into a conduit consisting of a corrugated hose connecting an ambient-air suction aperture in the vehicle body to an inlet stub of the air filter. This intake muffler dampens air noise generated at the suction zone within a narrow frequency band straddling the resonance frequency of the resonator.
German offenlegungsschrift 32 34 634 A1 discloses a resonator of a similar construction which is integrated directly ahead of the filter inlet stub. Two rows of apertures connect the filter inlet stub acting as the intake pipe of the intake muffler to the inside of the enclosing resonator. Both rows of apertures are arrayed in such manner that they cause .lambda./2 and .lambda./4 damping relative to the natural frequency of the inlet stub. While muffler effectiveness is improved thereby, its bandwidth is not.
German patent document 35 31 353 C2 discloses an intake muffler "plugged" with damping material and integrated into the inlet stub of a booster-air cooling device to be used in an internal combustion engine fitted with a booster (supercharger).
The intake mufflers of the state of the art illustrated above effectively dampen only within a narrow frequency band. Moreover, the muffler plugged with damping material are suitable only for systems having moderate superpressure. Mufflers damped with damping materials are unsuitable for a supercharger intake conduit.
It is known from International Patent Application WO 92/14922 A1 to construct a broadband intake muffler by connecting in parallel variously elongated side pipe resonators. Even though these resonators are made partly compact using labyrinths, this intake muffler is nevertheless still sufficiently bulky as to preclude practical use in automotive engineering.
It is already known, from European patent document EP 242 797 B1 as regards diffusers and from German offenlegungsschrift 41 34 408 A1 as regards a bypass resonator to construct intake mufflers with an effective broad bandwidth while averting bulk by using flaps and valves to produce matched damping systems which can be adjusted in relation to engine speed.
These systems incur the drawbacks of requiring a more or less complex regulating means, and additional installation space is required than for the initially described in-line resonators.
This state of the intake muffling art is faced with increasing ecological demands that motor vehicles shall reduce fuel consumption significantly. In particular, highly effective superchargers are inevitable to implement such goals.
The turbochargers presently used for such purposes operate at rotor speeds of nearly 200,000 rpm. Obviously turbochargers meeting such high requirements can practically remain economical only by trading off manufacturing tolerances. As a result, high acoustic radiation arises from such turbochargers, in particular the typical turbocharger "whistling" in the approximate frequency range of 2 to 4 kHz. In the process too, the boosting steps per se in the intake conduit also generate broadband radiated noise in the 4-6 kHz band, which is called "hissing".
Consequently, environmentally friendly motor vehicles shall require broadband absorbers of airborne noise in the foreseeable future in order to act as intake mufflers in motor vehicles using internal combustion engines charged by a supercharger and making it possible to effectively dampen the frequency band of 2 to 6 kHz.
Accordingly, it is an object of the invention to create a generally applicable broadband airborne-noise absorber which is also appropriate as an intake muffler for an internal combustion engine and which, while of minimal bulk that can be easily matched to specific requirements, allows effective damping of airborne noise uniformly over a broad frequency band. In particular, such an intake muffler shall be able to dampen noises acoustically generated and radiated by turbochargers in the approximate frequency band of 2 to 6 kHz to such an extent that for practical purposes they no longer can be heard either inside or outside the vehicle.